English incorporated Spanish el lagarto, the lizard. The noun lagarto is traced through reconstructed Vulgar Latin lacartus to Latin lacerta.
Lacartus is reconstructed, conventionally written *lacartus, rather than directly attested here. Latin lacertus is an alternative to lacerta, not another chronological card. The article is added at the phrase step.
Source
Merriam-Webster: alligatorWord History supplies el lagarto, both components, the reconstructed Vulgar Latin form and the Latin alternatives.